Howdy Dertonarm, and All
I had some time to speak to the old Egyptians (during my sleep last night), so here goes.
1) It is impossible to adjust overhang by moving the whole tonearm !!
THIS STATEMENT (ON ITS OWN) IS INCORRET. OVERHANG IS THE DISTANCE BETWEEN THE STYLUS AND THE CENTRE PIN WHEN THE TONE ARM IS IN LINE WITH THE CENTRE PIN (most tone arms do not allow physically to move this far, but the stylus' prescribed arc will exactly show it also (look at an LP12 set-up template and you'll see what I say is correct).
2) Overhang is a value that DEPENDS on the geometry of the given tonearm !!!
THIS STATEMENT IS CORRECT AS IT STANDS.
3)The geometry of ANY pivot tonearm has one foundation: the spindle-bearing distance (= mounting distance).
THIS IS ALSO CORRECT -- IF "GEOMETRY" MEANS "OVERHANG" IN THIS CONTEXT.
4) If you move the pivot of the bearing away from the specified value as given by the designer, the whole geometry of your tonearm is gone!
THIS IS ONLY CORRECT IF (AGAIN) "GEOMETRY" MEANS OVERHANG.
But since overhang is adjustable with EVERY arm, either by head-shell (oblong holes) or moving the arm-post it is NOT "gone" as you put it.
Here is the deal from the 2500 year old Egyptian.
First ask why do you have OVERHANG in the first place, and than also ask WHY do different arm specify DIFFERNT overhangs, and why do different so called 9" arms have not exactly the same distance between mounting hole and pivot, and pivot and centre pin?!
BECAUSE IT ONLY MATTER SO... MUCH!
All we do with these different measurements, is using the OVERHANG to ensure that the two NULL-POINTS (or zero tangential tracking error) are distributed to the "right" places (two) over the record!
Note: This "right" place does not even exist!
Every expert has his own idea of what's best, not just Baerwald, Linn, etc. etc. not even the Egyptians would know.
Why? Because it e.g. depends how wide you decide to 'spread' the two "Null-Points" i.e. the shorter the overhang the narrower the 'spread' (distance between the two points) since you have made the tone-arm's prescribed arc smaller. MORE overhang conversely gives you a wider spread. So there is some agreement on "more or less" where you want to allow the most and where the least error ---- and that's ALMOST all.
But not all things are quite equal:
The pivot to centre pin distance ALSO determines WHERE on the record your particular "null-point spread" would be! No good to have a zero tracking-error where there is NO GROOVE, (or better where there is MOSTLY no groove).
So having determined e.g. that it (null-point) is some 2 1/2" from the label (inside), then you figure it is say ~ 1/2 inch after the start groove --- you obtain by geometrical principal the overhang required for a GIVEN pivot to centre pin tone-arm measure.
That's why (amongst other things like alignmnet preference)not all overhangs are quoted the same (15, 17, 18 mm, etc.), because the pivot to centre pin distances are not all the same with 9" arms or 10.5" , 12" etc.!
So back to the geometry argument and how 'crewed up' the Egyptian says it gets.
Let's look at SME again, OK.
The "error" comes in by the arm-post (arm pivot) moving, to obtain a pre-determined overhang (given by SME set-up template). We now ever so slightly change the arc, by e.g. moving the arm post forward with e.g. a cart of shortish stylus mounting hole distance.
But HOW MUCH will that be? My current experience tells me within 1mm, of course it depends on the cart. Take a VERY odd one, the Dynavector 17D3 Karat Cartridge and it will get as way-out as it gets, perhaps 2mm short of the more average 9mm distance --- so I'll better not use that, unless I dig the resultung change in alignment.
Back to the +/- 0.5mm stylus mounting hole variation. Now if the distance is more, the 'null point spread' gets a bit wider and visa versa a bit narrower when the distance (stylus / mounting hole) gets less.
The question is: How much is "a bit"?
That "bit" is equal to the above mentioned flavours of overhang producing Linn, Baerwald and what not, alignments --- take your pick. Go with what sounds best for your system and your ears. I have tried (from one template alone) about four different overhang flavours, each one having it's pros and cons. The difference between them (resulting change of prescribed arc) is not more than a few mm of where these Null-point wind up on the record.
So what's the Egyptian say: Don't worry be happy --- unless you INSIST on a particular geometry (alignment) that is NOT a Linn (then don't by one) or a SME then don't either.
Both of these have FIXED head-shell holes / arm-pivot to head-shell to centre pin distances. And both have been going for longer than ANY of the all current offerings also, funny no?
Linn sells their own carts and by that they make sure you got it right, SME hope you get the right cart +/- since they know it is NOT THAT critical after all, where you null-point are. 1-2 mm this way or that way will be hard to tell, even with bats-ears I guess.
Greetings,
Axel
